1. Technical Field
The present invention relates to a semiconductor device.
2. Related Art
In the development of an advanced CMOS (complementary MOS) device having a small size, a driving current deteriorates due to the depletion of a polysilicon (poly-Si) electrode and a gate leakage current is increased due to a reduction in the thickness of a gate insulating film. Therefore, a composite technique has been examined in which a metal gate electrode is used to prevent the depletion of the electrode and a gate insulating film is made of a high-dielectric-constant material to increase a physical thickness, thereby reducing the gate leakage current.
For example, a pure metal, a metal nitride, or a silicide material has been examined as a material forming the metal gate electrode. However, in all the cases, the threshold voltages (Vth) of an N-type MOSFET and a P-type MOSFET need to be set to appropriate values.
In order to obtain a threshold voltage Vth of ±0.5 eV or less in the CMOS transistor, the gate electrode of the N-type MOSFET needs to be made of a material having a work function that is equal to or less than the mid-gap (4.6 eV) of a Si, preferably, 4.4 eV, and the gate electrode of the P-type MOSFET needs to be made of a material having a work function that is equal to or more than the mid-gap (4.6 eV) of a Si, preferably, 4.8 eV.
In order to achieve the above, a method has been proposed in which different kinds of metal materials or alloys having different work functions are used for the electrodes of the N-type MOSFET and the P-type MOSFET to control the threshold values of the transistors (dual metal gate technique). For example, as a first related art, in International electron devices meeting technical digest, 2002, p. 359, is disclosed a structure in which the work functions of a Ta electrode 23 and a Ru electrode 24 formed on a SiO2 film 20 are 4.15 eV and 4.95 eV, respectively, and a work function of 0.8 eV between the two electrodes can be modulated, as shown in FIG. 17A.
In recent years, a technique related to a silicide electrode obtained by completely siliciding a poly-Si electrode with, for example, Ni, Hf, or W has drawn attention. For example, as a second related art, in International electron devices meeting technical digest, 2002, p. 247 and International electron devices meeting technical digest, 2003, p. 315, is disclosed a technique in which the SiO2 film 20 is used as a gate insulating film, Ni silicide electrodes (a P-doped NiSi electrode 25 and a B-doped NiSi electrode 26) obtained by completely siliciding a poly-Si electrode doped with impurities, such as P or B, with Ni are used as gate electrodes, and the work functions of the electrodes are modulated by a maximum of 0.5 eV, as shown in FIG. 17B. This technique has characteristics that, after a high-temperature heat treatment is performed to activate the impurities of a source/drain diffusion layer 9 of the CMOS, it is possible to silicide the poly-Si electrode. This technique is well matched with the CMOS process according to the related art. In addition, in International electron devices meeting technical digest, 2002, p. 247 and International electron devices meeting technical digest, 2003, p. 315, when a SiON film is used as the gate insulating film, the work functions of NiSi and NiSi2 for the gate electrodes are about 4.6 eV and 4.45 eV, respectively.
In the second related art, the gate insulating film is the SiO2 film 20 or the SiON film. However, a technique for combining the technique that uses the high-dielectric-constant insulating film as the gate insulating film to reduce the leakage current with a metal gate technique is also important. As a third related art, in International electron devices meeting technical digest, 2004, p. 91, is disclosed a method that makes the contents of Ni of the Ni silicide electrodes in the N-type MOSFET and the P-type MOSFET different from each other to obtain work functions required for the N-type MOSFET and the P-type MOSFET. For example, a technique has been proposed in which a HfSiON film 21 is used as a gate insulating film and Ni silicide electrodes (a NiSi or NiSi2 electrode 27 or NiSi is used in the N-type MOSFET and a Ni3Si electrode 28 is used in the P-type MOSFET) including different amounts of Ni are used as the gate electrodes to modulate the work functions of the electrodes by a maximum of 0.4 eV, as shown in FIG. 18A.
In addition, a technique has been proposed which increases the modulation width of the work function using a Ni silicide gate electrode on a high-dielectric-constant gate insulating film as a base. For example, as a fourth related art, in International electron devices meeting technical digest, 2005, session 27, p. 6, is disclosed a technique in which a HfO2 film 22 is used as a gate insulating film and gate electrodes obtained by adding different kinds of metal materials or the silicides thereof to Ni silicide (an Al-added NiSi electrode 29 is used in the N-type MOSFET and a NiPtSi electrode 30 is used in the P-type MOSFET) are used, thereby obtaining a work function of 4.2 to 4.3 eV in the N-type MOSFET and a work function of 4.85 eV in the P-type MOSFET, as shown in FIG. 18B.